Fossil fats suggest animal life got started later than previously thought

Rhizaria algae is now believed to be responsible for fossil fat molecules that have confused scientists about when complex life first arose(Credit: Fabrice Not)

In the world of palaeontology, new discoveries often proclaim things are older than we previously thought – after all, once an age record has been broken, it only becomes newsworthy again when something even older turns up. But new research out of the Max Planck Institute has found that animals probably arose later than is currently believed.

The further back we trace life on Earth, the murkier our origin story gets. The current story goes that single-celled organisms appeared pretty quickly after Earth first formed, and dominated the planet for three billion years or so. Multicellular life, which eventually became today's animals (and us), has been definitively dated by fossils back to about 560 million years ago.

But there has been fossil evidence to suggest that animals are much older than that. A few years ago, fossil fat molecules were discovered in rocks that put them at 645 million years old. These were originally attributed to sea sponges, which would make them the oldest known animals by quite a wide margin.

"But the first unambiguous sponge fossils ever found, shaped as needles or spicules, are 100 million years younger than these old sponge molecules," says Benjamin Nettersheim, first author of the study. "That's a huge gap, the molecules and spicules cannot both be right."

In the new study, the Max Planck researchers found another explanation for the fossil fats. The team discovered the same kind of fat molecules in Rhizaria algae, a single-celled organism that has its origins as far back as 770 million years. While it's possible that both Rhizaria and sponges left the fossil fats behind, the team says it's more likely that the algae worked alone. To leave the molecules in those amounts, sponges would've had to be pretty widespread – which in turn means that there should be older fossils of them.

"From an ecological perspective Rhizaria just make so much more sense," says Nettersheim. "If sponges were the source, they would have needed to occur in massive abundances, thriving virtually everywhere, even in oxygen-depleted waters where sponges typically cannot survive."

The new discovery smooths out the story of life on Earth, and fits in nicely with another recent fossil fat study by Nettersheim and co-author Christian Hallmann. Roughly 700 million years ago, an extreme ice age gripped the planet, in a phase sometimes called Snowball Earth. As this began to melt about 650 million years ago, huge amounts of nutrients were washed into the oceans, and the sudden influx of food gave rise to algae, like Rhizaria – which of course left their fat molecules behind in fossils.

The age of algae probably continued for a few dozen million years, before more complex life, such as Dickinsonia, burst onto the scene about 560 million years ago.

"In geological terms, this is right before the onset of the Cambrian Explosion of complex lifeforms 540 to 550 million years ago, and the new timing now provides us with a coherent sequence of events," says Hallmann.